Camshaft Position Sensor B Circuit Range/Performance (Bank 2)
The camshaft position sensor on the right side of the engine isn't sending signals in the expected range, like a clock hand moving too slow or too fast to tell time properly. This prevents the engine computer from syncing fuel injection and ignition timing correctly.
The ECU monitors voltage and frequency signals from the camshaft position sensor on Bank 2 to determine cam timing and valve overlap. If the signal arrival time drifts beyond expected thresholds or voltage stays out of range, the ECU logs this fault. The sensor typically generates a square wave or analog signal that the ECU compares against crankshaft timing references.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Signal Voltage | 0.5–4.5V (typical range) | Below 0.2V or above 4.8V |
| Cam Timing Deviation | Within ±5° of expected position | Exceeds ±5° correlation to crank |
Code P0391 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0391 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.